1. Field of the Invention
The invention relates generally to the field of digital subscriber lines. More particularly, the invention relates to asymmetric digital subscriber lines. Specifically, a preferred implementation of the invention relates to asymmetric digital subscriber lines over long loops.
2. Discussion of the Related Art
Asymmetric Digital Subscriber Line (ADSL) technology provides one of the most efficient and popular methods used for providing connectivity between a subscriber's premises and the Central Office of the serving telephone company. The primary use of ADSL today is for providing subscribers with efficient, always-on, high-speed access to the Internet. Other services, such as telephony, can also be provided over the same digital subscriber line.
Typically, communication between an end-user and an Internet Service Provider (ISP) is characterized by a relatively high bit-rate requirement in the downstream (from ISP to end-user) or download direction, and a relatively low bit-rate requirement in the upstream (from end-user to ISP) or upload direction. ADSL technology was developed to address this need and available services provide an upstream bit-rate typically in the range of 128 kbps to 384 kbps and a downstream bit-rate typically in the range of 384 kbps to 6 Mbps. A typical premium ADSL service provides 256 kbps upstream and 1.5 Mbps downstream.
A problem with this technology has been the decreasing bit-rate capacity of ADSL with increasing loop length. This is primarily because the signal is attenuated to a greater extent with increasing loop length. This attenuation could, in principle, be offset by increasing the transmit signal power. In particular, since the attenuation increases with frequency, the transmit power spectral density could be shaped to ensure greater power at higher frequencies. However, there are regulatory constraints that prevent this arbitrary power spectral shaping. In particular, the transmit power spectral density is constrained to meet a “mask” which limits the signal power at the transmitter (and at any intermediate point in the loop) as a function of frequency.
Additionally, the longer the loop, the greater the possibility of having induced interfering signals from such diverse sources as AM radio broadcasts and from signals present in other loops that share the same binder group (subscriber cable is constructed with several loops, typically 25 pairs, contained in a common sheath or “binder group”). The modems utilize digital signal processing techniques to achieve the digital transmission over the pair of wires constituting the subscriber loop. Thus, inherently, there is an analog-to-digital conversion process and the added quantization noise further, negatively, impacts the supportable bit-rate.
In an attempt to solve the above-discussed problems, one approach has been developed involving improving the transmission bit-rate for long subscriber loops utilizing extenders (often called repeaters), which are placed in the loop plant between the Central Office and the subscriber premises. However, a disadvantage of this approach is the need for additional Central Office equipment.
Another disadvantage of this approach has been relatively high cost. Therefore, what is also needed is a solution that meets the above-discussed requirements in a more cost-effective manner.
Heretofore, the requirements of increasing downstream bit-rates for long ADSL loop lengths utilizing methods and apparatus deployed at the subscriber premises, obviating the need for additional Central Office equipment, eliminating the possibility of spectral mask violations associated with the improper deployment of a mid-span repeater, and eliminating the need for outside plant construction referred to above have not been fully met. What is needed is a solution that addresses this all of these requirements.